For quite some time now, the art has been aware of the advantages of modulation techniques in which the carrier is not transmitted. With the advent of phase-locked loops and techniques associated therewith the demodulation of suppressed carrier signals has been greatly simplified.
Briefly, a conventional phase-locked loop may include a phase detector, a low pass filter and voltage controlled oscillator. The controlled oscillator phase makes it capable of locking or synchronizing with an incoming signal. If the phase relationship changes, indicating the incoming frequency is changing, the phase detector output voltage increases or decreases just enough to keep the oscillator frequency the same as the incoming frequency, preserving the locked condition.
In many cases in suppressed carrier applications the suppressed carrier frequency actually changes due to phase shifts, downward shifts or other effects and the loop provides the demodulator with an excellent means for tracking this change in frequency to provide effective demodulation.
However, under certain circumstances the loop can be side locked. Side lock is an anomalous locking mode in which the phase-locked loop output frequency is not the desired lock frequency. For the conventional phase-locked loop, side lock is a stable condition. The difficulty is, of course, that in side lock the voltage controlled oscillator is not oscillating at the correct frequency to perform the necessary demodulation.
A discussion of side lock is found in Burst Synchronization of Phase-Locked Loops, by Leonard Schiff, found in IEEE Transactions on Communications, Volume COM-21 No. 10, October 1973 at pp. 1091 through 1099. This article teaches that side lock may be prevented by reducing the allowable offset between the carrier frequency and the voltage controlled oscillator natural frequency to be less than half the burst synchronizing rate. Of course, due to frequency uncertainty in longdistance transmissions, carrier frequency drifts and the like this may not be a practical alternative. As a further alternative the author employs a frequency discriminator to provide a voltage proportional to the difference between the carrier frequency and the lock frequency, where a carrier frequency is available. Again, depending upon the application this may or may not be practical.
A severe test of the phase-locked loop capabilities is found in demodulating PSK signals especially where there are plural phases. The prior art teaches that the carrier may be recreated by first multiplying the received signal, in frequency, by a number equal to the number of phases of modulation. The resulting signal is then applied to a phase-locked loop whose voltage controlled oscillator has natural frequency of N times the carrier (NFc). The output of the phase-locked loop is desirably then N times the carrier frequency (NFc) which, after division by N can be used for synchronous detection purposes. Such an arrangement is illustrated in FIG. 1.
The difficulty with this approach can be illustrated by noting that the spectral energy distribution at point A (in FIG. 1) is that shown in FIG. 2A. FIG. 2A shows the multiplied carrier frequency at a frequency of (NFc) and also shows a plurality of side bands spaced from either the carrier or another side band by the modulation rate R.sub.s of the PSK signal. If R.sub.s is relatively small, compared to the frequency uncertainty of the received carrier it may be difficult, or impossible, to design the phase-locked loop components to avoid side lock.